This application claims the priority of German Application 19925127.4, filed Jun. 2, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an antenna arrangement in motor vehicles. Antenna arrangements in motor vehicles are known for various operational purposes. For example, a combination antenna is known from Funkschau 2/95 [Radio show], pages 60-62, in which a first antenna is formed as an annular slot antenna for a mobile radio network, inserted in an insulating form between a bodywork cutout and a conductor surface in it, with a surface antenna for a satellite navigation system also being arranged on the conductor surface. Active rod antennas and windshield antennas with elongated conductor tracks along windowpane rims are also known, as is the use of line sections running along cable harnesses.
The present invention is based on the object of specifying a simple antenna arrangement which can be produced cost effectively and can be integrated in the vehicle structure.
The invention is distinguished by a compact antenna element in the form of a conductive antenna surface which is insulated from ground, which is a reference-ground for the radio-frequency signals. The compact element is positioned a short distance from this ground, with the preferably essentially planar antenna surface of the ground facing a surface edge, which is referred to in the following text as the active surface edge, but otherwise pointing away from the ground, and, in particular, with the surfaces not being opposite one another. The ground is preferably formed by a part of the vehicle bodywork.
The antenna surface can advantageously be applied as a thin conductive layer on a dielectric supporting surface, for example bonded on or deposited as a film. The antenna surface does not need to be strictly planar, so that a large choice of suitable surfaces is available on the vehicle.
The antenna surface is advantageously positioned in the region of the window openings of the vehicle bodywork, in particular in the rear window or preferably in the front window region. The windshield surface itself or, according to another advantageous embodiment, a dielectric housing part of a functional unit that is independent of the antenna arrangement, may be used, for example, as the dielectric support. The functional unit is, as a result, advantageously used without requiring an additional support for the antenna surface. If the antenna surface is arranged in the rim region of a windowpane surface, the antenna surface is advantageously covered by a sheet, with a position close to a further functional unit advantageously being chosen once again, and a common cover being provided for the antenna surface and the further functional unit.
According to a particularly advantageous embodiment, the further functional unit may be an optical rain sensor arranged in the upper region of the windscreen.
The arrangement of the antenna surface in the upper rim region of a window opening has the further advantage of particularly low screening and shadowing by conductive bodywork parts, so that this advantageously results in good accessibility from all sides by a portable radio transmitter, in particular a radio key for a locking system which can be remotely operated by radio.
The radio frequency receiver, which is connected to the antenna surface via a connecting lead, is advantageously arranged a short distance away from the antenna surface, as a result of which a short connecting lead may be used, which need not have specific radio-frequency characteristics in the same way, for example, as a coaxial lead. The receiver is preferably shorted on one side to ground for radio frequency, without making any direct electrical contact with this ground, for example by the receiver assembly being arranged a short distance away from the bodywork and being capacitively and/or inductively connected to the ground and to the reference-ground potential that is present on it. The position of the antenna surface in an upper region of a window opening is also particularly advantageous for this purpose, because the roof surface is particularly suitable for use as ground, and the receiver assembly can easily and inconspicuously be integrated in the inner lining of the roof surface and/or in an additional functional group, which is frequently present, under the vehicle roof in the vicinity of the front window, for example a ceiling light, a roof-mounted control unit, etc. The ground coupling of the receiver for RF signals may also be added to by coupling into a cable harness.
In one preferred embodiment, the antenna surface is arranged approximately at right angles, and with its active surface edge parallel to an essentially straight edge of a ground which is a short distance away, in particular a bodywork panel. The distance, or the minimum distance in the case of edges which are not straight or not parallel, between the active surface edge and the opposite edge of ground is preferably greater than 0.005 xcex, in particular greater than 0.01 xcex and preferably less than 0.05 xcex, in particular less than 0.025 xcex, where xcex is the operating wavelength of the antenna arrangement. The size of the antenna surface in a direction parallel to the shortest connection between the active surface edge of the antenna surface and the opposite edge of ground is preferably greater than 0.02 xcex, in particular greater than 0.04 xcex and preferably less than 0.2 xcex, in particular less than 0.1 xcex. In a direction at right angles to this, if the edges of the antenna surface and earth surface are parallel and opposite, that is to say in particular in a direction parallel to these edges, the applicable sizes for advantageous dimensions of the antenna surface are essentially the same as those for the direction described above. The ratio of the transverse dimensions of the antenna surface in the two directions is advantageously between 0.2 and 5.
The antenna arrangement is preferably used in a radio system having an operating wavelength between 0.2 m and 2 m. The antenna arrangement can be used particularly advantageously for a locking system which can be operated by radio and has a radio key as a portable transmitter.
In a first preferred embodiment, the antenna surface is formed as a continuous, uniform, conductive surface. However, this function is maintained even if the surface is interrupted, for example being in the form of a grid, provided the width of the openings is very small in comparison with the operating wavelength. The antenna surface may also be structured in some other way, for example in spiral form or in the form of separate strips parallel to the edge, provided the distances between separate sections is very short in a direction parallel to the shortest connection of the active surface edge and the opposite edge of the earth surface, and the antenna impedance is not unacceptably adversely affected. In some circumstances, the antenna surface may be structured in order to adjust the impedance.
Another particularly advantageous measure for setting a desired impedance, in particular for impedance matching between the antenna surface and the receiver input, provides for reactive elements to be inserted between the antenna surface and the receiver input, in order to influence the impedance. According to one preferred embodiment for this purpose, such reactive elements are in the form of conductive surface structures between the antenna surface and the connecting lead to the receiver.
The connecting lead is advantageously connected into a section which is offset with respect to the centre of the surface element in the direction of the ground, preferably close to the active surface edge, and possibly with the interposition of the aforementioned surface structures of the reactive elements.
The length of the connecting lead is advantageously short in comparison with the operating wavelength, and is preferably shorter than 0.5 xcex, in particular shorter than 0.25 xcex.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.